Among conventional gas turbine engines, there exist both the type that requires cooling by the circulation of relatively cool atmospheric air, and the type which requires no such cooling. Of the air-cooled type relatively cool air drawn from the main air stream of the engine is circulated past a forward air seal mounted onto a first stage turbine disk. The air is then provided to a cooling system built-in to turbine blades mounted onto the turbine disks, and which serves to cool the turbine blades prior to exiting into the exhaust gas flow of the engine.
Typically, the cooling system comprises a plurality of small bore cooling ducts formed in the turbine blades, which permit passage of the air therethrough. Due to the small diameter of the cooling ducts they are prone to becoming clogged in a dusty or sandy environment unless the dust and sand is removed from the air flow prior to its reaching the cooling system. The clogging of the turbine blade cooling system in this way can significantly reduce the operational life of the engine.
Conventional sand separators may be divided into main categories, namely, external and internal separators, both of which are used, for example, with the General Electric T700 engine. A full description thereof may be found in a report entitled "The T700-GE-700 Engine Experience in Sand Environment", presented by Z. J. Przedpelski at the Rotary Wing Propulsion System Specialist Meeting, Williamsburg, Va., U.S.A., Nov. 16-18 1982, and published in the Journal Of American Helicopter Society, Vol. 29, No. 4, Oct. 1984, pp. 63-69.
External sand separators are typically provided so as to reduce the erosion by sand particles of compressor and turbine blades in the engine. These separators include a blower for drawing sand particles from the air intake and for expelling them out into the atmosphere, such that the remaining air, which is subsequently provided to the engine, is generally free of sand particles. Such systems do not, however, prevent the entry of the very fine sand and dust particles from entering the engine and blocking the very narrow cooling air ducts in the turbine blades.
An internal separator, also used with the T700 engine, is located within the engine, downstream of the engine compressor. A portion of the air leaving the compressor flows to the combustion chamber, while a further portion is directed towards the turbines so as to cool them. The engine is configured such that centrifugal forces acting on the sand particles cause them to be driven into the air intake of the combustion chamber, leaving that portion of the air flow to be directed to the turbine blades, with a reduced sand content.
It has been found, however, that the particle removal capability of these systems is only partial, and that even when they are used, blockage of the turbine blade cooling ducts may occur, nonetheless, after a relatively short time. Additionally, the removed particles are allowed to accumulate within the engine, thus requiring its periodic stripping down so as to allow removal of the accumulated material.